Technical Abstract: The paper objective is to present an overview of the greenhouse gas research results from tillage and N fertility studies conducted by USDA-ARS on irrigated cropping systems near Fort Collins, Colorado from 2002–2010. Within agronomic N rates needed to optimize irrigated crop yields in the Central Great Plains, a linear increase in growing season nitrous oxide (N2O-N) emissions was observed from a clay loam soil with increasing fertilizer N rate. Averaged over a 5 yr period (2002-2006), N2O-N emissions were greater from a conventional-till continuous corn (CT-CC) production system than from a no-till continuous corn (NT-CC) production system. Adding soybean or dry bean to the rotation resulted in increased growing season N2O-N emissions during the corn year of the corn-bean rotation when compared to CT-CC or NT-CC systems. Fertilizer N source effects on growing season N2O-N emissions were also evaluated. Emissions were monitored from several inorganic N fertilizer sources (urea, ESN1, Duration III1, SuperU1, UAN, UAN+AgrotainPlus1, UAN+Nfusion1) from 2007-2010. Comparing N2O-N emissions (2007-2008) from urea and ESN in CT-CC and NT-CC cropping systems, N2O-N emissions were not different between urea and ESN in the CT-CC system; however, N2O-N emissions were significantly less with ESN than with urea in the NT-CC system. Emissions from urea and ESN were significantly less in the NT-CC system than in the CT-CC system. Growing season N2O-N emissions from SuperU (a stabilized N source) was significantly less than from urea in NT corn-barley and NT corn-dry bean rotations in 2007 and 2008. The effects of several enhanced-efficiency N sources on growing season N2O-N emissions were compared in a NT-CC system in 2007 and 2008. The enhanced-efficiency N fertilizers and UAN reduced growing season N2O-N emissions compared to urea, with the SuperU and UAN+AgrotainPlus having lower emissions than UAN. In 2009 and 2010, the enhanced efficiency N sources were compared in a strip-till (ST), irrigated continuous corn production system. Averaged over 2 yr, all N sources had significantly lower growing season N2O emissions than dry granular urea, with UAN+AgrotainPlus having lower emissions than UAN. The check treatment with no N applied had the lowest level of emissions in all years. Cumulative increases in daily N2O-N fluxes were more rapid for urea and UAN than the other N sources following N fertilizer application. The enhanced efficiency fertilizers (polymer-coated, stabilized, and slow release) sources showed potential for reducing N2O-N emissions in semi-arid, irrigated cropping systems in the Central Great Plains.